This invention relates to a carburetor for a two-cycle air scavenging and stratified internal combustion engine and more particularly to a carburetor with a sealing-and-bearing assembly for a butterfly control valve.
Carburetors for small two-cycle or two-stroke air scavenged internal combustion engines commonly used for hand held power tools such as chain saws, weed trimmers, leaf blowers and the like are known to have both an enriched fuel-and-air mixing passage and a primary air flow passage which both communicate between a clean air source at atmospheric pressure such as an air filter and a crankcase and combustion chamber respectively of the two-cycle engine. During initial acceleration of the two-cycle engine, a butterfly-type throttle valve within the fuel-and-air mixing passage opens to provide an enriched fuel-and-air mixture to the engine. Toward the end of the acceleration, or after acceleration, a butterfly-type control valve within the primary air flow passage will open permitting clean air to flow through the passage to scavenge exhaust gases from the combustion chamber and provide a relatively leaner mixture of fuel-and-air in the combustion chamber of the running engine. This delay in the opening of the control valve provides a richer mixture of fuel-and-air to enhance acceleration and exhaust scavenging and a leaner mixture to reduce hydro-carbon exhaust emissions during steady state running conditions of the engine. The clean air may also provide a stratified arrangement or layering in the combustion chamber of exhaust gases, fresh air and a fuel-and-air mixture.
Both valves have rotating shafts which are mechanically interlinked or cammed to provide the delayed and synchronized operation of the control and throttle valves. The shaft of the control and throttle valves rotate directly against the body of the carburetor. The clearance between the body and each rotating shaft is slight or small enough to minimize or prevent the ingress or leakage of atmospheric air into the fuel-and-air mixing passage which would otherwise alter the fuel-and-air mixture ratio degrading the operation of the running engine. For two-cycle engine applications, the fuel contained within the enriched fuel-and-air mixture is usually a combination of gasoline and oil, typically mixed at a ratio of 50:1. The oil not only lubricates the moving parts within the crankcase but also lubricates the throttle valve shaft to carburetor body interface.
A carburetor for a scavenged and stratified two-stroke internal combustion engine which has a fuel-and-air mixing passage and a separate primary or clean air passage with a butterfly-type control valve with a seal and bearing assembly for the actuator shaft of the valve. The valve shaft extends transversely through the primary air passage and is rotatably received in a pair of spaced-apart coaxial cylindrical bores in a body or housing containing the primary air passage and immediately adjacent the air passage with a relatively close fit to at least in part provide an air seal between the shaft and the primary air passage. The valve shaft is also journalled for rotation in a separate bearing or bushing inserted into a counterbore adjacent at least one of the cylindrical bores and terminating short or outboard of the primary air passage to prevent excessive wear of the adjacent cylindrical bore and in part providing seals between the shaft and the body or housing to at least in part prevent air leakage into the primary air flow passage. This seal and bearing configuration also provides a better fit between the closed control valve plate and the air passage to reduce unwanted air flow between them when the control valve is closed.
Preferably, the carburetor has the primary air passage and its associated control valve in one body which in assembly is attached to a separate housing containing the enriched fuel-and-air mixing passage and its associated throttle valve assembly. The control valve of the air flow passage is operably connected with the throttle valve so that when accelerating the engine from idle, the throttle valve initially opens and thereafter the control valve in the air flow passage opens in synchronization with the throttle valve. Preferably, the control valve is operably connected to the throttle valve by a mechanical linkage or cam arrangement.
Preferably, in one form the primary air flow passage and associated control valve are in a separate body attached to one end of a main housing containing the enriched fuel-and-air mixing passage, associated throttle valve, and a pump supplying fuel to a metering chamber. In a second form, the body containing the air flow passage and control valve is attached to the fuel metering side of the main housing of the carburetor. In a third form, the primary air flow body is attached to a fuel pump side of the main housing of the carburetor. Attaching the body of the primary air flow assembly to either the fuel metering side or the fuel pump side of the main housing of the carburetor eliminates the need for a separate cover for the fuel metering system or pump and significantly reduces the size of the carburetor.
It has been discovered that since the rotating shaft of the control valve in the primary air flow passage of the carburetor is not exposed to the enriched fuel-and-air mixture in use it is not lubricated and no capillary seal is provided between the shaft and the body. Thus, without this sealing-and-bearing assembly, the body wears away and galls the rotating shaft of the control valve resulting in seizure and failure, and enlargement of the clearance between them which allows air and damaging contaminates to leak into the primary air flow passage between the worn shaft and body interface, thereby degrading operation of the engine.
Objects, features and advantages of this invention include a sealing-and-bearing assembly which eliminates valve control shaft wear and air ingress into the primary air flow passage along the rotating shaft of the control valve while minimizing clean air flow bypass around the plate of the control valve when closed within the primary air flow passage thereby providing a reliable and smooth running engine which is significantly impervious to dirt and debris. Additional advantages are a reduced number of parts, a reduction in carburetor size, and a design which is easily incorporated into existing carburetors. This design improves engine performance and is relatively simple and economical to manufacture and assemble, and in service has a significantly increased useful life.